Grating Coupler Spectrum Fitter
Paste a loopback scan and get the fixture numbers your de-embedding needs: center wavelength, per-coupler insertion loss, 1 dB and 3 dB bandwidths from a parabola fit in dB, and an analysis of the residual ripple, whose period reveals where the parasitic Fabry-Perot cavity lives. Companion procedure: De-embedding coupler loss with reference structures.
Two columns per line: wavelength in nm, then transmission. Comma, tab, or space separated. dB is detected automatically (or force it below).
The example is a synthetic back-to-back scan: 4.8 dB per coupler at 1547 nm, 16 nm 1 dB bandwidth, 0.4 dB of Fabry-Perot ripple from a 0.89 mm parasitic cavity, and 0.05 dB of noise. Near its peak a grating coupler is parabolic in dB, so the fit runs on the points within your chosen window of the maximum, then re-centers itself once on the fitted peak.
Per-coupler numbers assume the couplers in the path are identical; on one die that is typically good to a few tenths of a dB. Procedure, error budget, and the worked ripple example: De-embedding coupler loss with reference structures.
These validation rows run in your browser on every load: the fitter is exercised on synthetic scans with known answers, including ripple-period recovery and the refusal path for peakless data.
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